450 
THE TERTIARY VOLCANOES 
BOOK VIII 
altitude, as described by Mr. Hardman, present great difficulties in the 
attempt to regard them as the sediments of a Pliocene lake. A more 
recent examination of the ground by Mr. Clement Eeid of the Geological 
Survey has led that able observer to believe that two totally different 
groups of strata at Lough Neagh have been confounded. He noticed the 
Mytilus-claj to be a dark blue mass full of derived boulder-clay stones, and 
yielding Mytilus eclulis and seeds of a sedge. This deposit cannot be 
Pliocene, but must be of Glacial or post-Glacial age, possibly contemporary 
with the Clyde beds. The junction of this clay with the pipe-clays is not 
at present seen, but the lithological contrast between the two groups of 
strata is so strong as to indicate their independence of each other. Mr. 
Eeid found the white, red and mottled pipe-clays with their masses of 
lignite to present a strong resemblance to the Bagshot group in the 
Tertiary series. It is possible, as already suggested, that the pipe-clays and 
lignites may belong to the sedimentary zone that separates the lower and 
upper basalts of Antrim. At all events they furnish no proof of any Plio- 
cene lake, and may not indicate more than a deeper part of the depression 
in which the tuffs, lignites and iron-ore were laid down. 
The existence of the Mytilus - clay shows that in Glacial or post-Glacial 
times the valley of the Bann was a strait or fjord into which the sea 
entered. Thick masses of drift have been laid down all round and over 
the depression now occupied by Lough Neagh, insomuch that had any 
older lake existed here in Glacial times, it could hardly have escaped being 
filled up. 
The observer, who from one of the basalt-heights looks down upon the 
expanse of Lough Neagh and the broad peat-covered plain that continues the 
level platform of the lake-surface down the valley of the Bann, cannot but 
be impressed with the size of this wide hollow in the heart of the Antrim 
plateau, and with the evident continuity of the whole depression from the 
lake to the sea. If he be a geologist, he will be further struck by the fact 
that while the Chalk and other older rocks appear from under the basalt- 
escarpments all round the plateau, at heights of many hundred feet above the 
sea, the floor of this wide hollow is entirely covered with basalt. Had the 
depression been merely due to denudation, the rocks that underlie the 
volcanic series would have been exposed to view. The base of the basalts 
which, on either side of the depression, is often more than 1000 feet above 
the sea-level, sinks below that level in the hollow of the Bann and Lough 
Neagh. 
This inequality of position may have been partially brought about by 
faults like those around Lough Neagh, and may thus have been begun long 
before the Glacial period. But it appears to me to be mainly due to a 
wide subsidence, of which the axis ran in a N.N.W. and S.S.E. direction 
from the present coast up the valley of the Bann and the basin of Lough 
Neagh to beyond Portadown. 
We may conceive that after the cessation of the outflows of basalt, the 
territory overlying the lava-reservoir that had been emptied would tend to 
